It is known to utilize certain compounds or mixtures of compounds in acidic solutions that are utilized for cleaning or pickling metal surfaces to remove therefrom unwanted oxides, scale and other undesirable corrosion products. Such compounds reduce the tendency of the acidic cleaning solution to corrode the metal surface without interfering with the cleaning operation performed by the solution. Compounds that function in this manner are generally referred to as “acid inhibitors”. In the absence of acid inhibitors, an acidic metal cleaning or pickling solution can cause significant base metal loss and also damage to the metal surface as a result of excessive hydrogen exposure.
The corrosion of metal surfaces by acidic cleaning solutions is caused by the acids present in the solution. Acids generally used in such metal cleaning or pickling solutions are the so-called “non-oxidizing” acids. Included in this category are inorganic (mineral) acids such as hydrochloric acid, phosphoric acid, sulphamic acid, sulphonic acid and sulfuric acid and organic acids such as acetic acid, citric acid, formic acid, glycolic acid and oxalic acid.
The acid component of the cleaning solution is effective in removing undesirable deposits from metal surfaces, but unfortunately it also tends to attack and corrode the base metal. Such corrosion is obviously very undesirable. To counteract the corrosive effects of the acid, acid inhibitors are added to the cleaning solution.
An effective inhibitor must disperse throughout the pickling solution in low concentrations, must suppress hydrogen evolution, and must not leave excessive smut or residual film on the surface of the metal. It must also maintain effectiveness over a range of acid and iron concentrations and temperatures, with such effectiveness being long lasting so that the metal pickling or cleaning solution need not be frequently discarded or replenished.
Many types of acid inhibitor compositions are known in the art, with several being available commercially. However, in many cases such formulations contain undesirably high concentrations of certain substances such as formaldehyde or acetylenic alcohols which are toxic and/or flammable and thus are subject to strict regulation due to environmental, health and safety concerns. Further, it is desirable for cost and convenience reasons to market such acid inhibitor compositions in the form of concentrates that are diluted and combined with aqueous acid solutions to prepare a metal pickling or cleaning solution. Alternatively, such concentrates are used to replenish working metal pickling or cleaning solutions that have become depleted or that no longer have the desired degree of effectiveness with respect to acid inhibition. Acid inhibitor concentrates must remain stable over prolonged periods of time so that they may be safely stored until being combined with aqueous acid to form or replenish a metal pickling or cleaning solution. That is, the concentrate should remain a homogeneous solution (e.g., no phase separation or precipitation of solids) and should not deteriorate or degrade in effectiveness to a significant extent. Moreover, the solutions prepared from such concentrates must meet stringent customer requirements with respect to cost and performance (e.g., inhibition of metal etching), both immediately and over time (e.g., as iron levels in the solution increase upon continued use of the solution).
Further improvements in the art of acid inhibitor concentrates and metal cleaning and pickling solutions would therefore be desirable.